Electron beam apparatuses, in particular a scanning electron microscope (also referred to as SEM below) and/or a transmission electron microscope (also referred to as TEM below), are used to examine objects (also referred to as specimens) in order to obtain knowledge in respect of the properties and behaviors of the objects under certain conditions.
In an SEM, an electron beam (also referred to as primary electron beam below) is generated by means of a beam generator and focused on an object to be examined by way of a beam-guiding system. An objective lens is used for focusing purposes. The primary electron beam is guided in a raster-shaped manner over a surface of the object to be examined by way of a deflection device. Here, the electrons of the primary electron beam interact with the object to be examined. In particular interaction particles and/or interaction radiation is/are generated as a result of the interaction. By way of example, the interaction particles are electrons. In particular, electrons are emitted by the object—the so-called secondary electrons—and electrons of the primary electron beam are scattered back—the so-called backscattered electrons. The interaction particles form the so-called secondary beam and they are detected by at least one particle detector. The particle detector may generate detector signals which are used to generate an image of the object. The image is displayed on a display device, for example a monitor. An imaging of the object to be examined is thus obtained.
By way of example, the interaction radiation is x-ray radiation or cathodoluminescence. It is detected for example with a radiation detector and is used in particular for examining the material composition of the object.
In the case of a TEM, a primary electron beam is likewise generated by means of a beam generator and focused on an object to be examined by means of a beam-guiding system. The primary electron beam passes through the object to be examined. When the primary electron beam passes through the object to be examined, the electrons of the primary electron beam interact with the material of the object to be examined. The electrons passing through the object to be examined are imaged onto a luminescent screen or onto a detector—for example in the form of a camera—by a system comprising an objective lens. By way of example, the aforementioned system additionally also may comprise a projection lens. Here, imaging may also take place in the scanning mode of a TEM. As a rule, such a TEM is referred to as STEM. Additionally, provision can be made for detecting electrons scattered back at the object to be examined and/or secondary electrons emitted by the object to be examined by means of a further detector in order to image an object to be examined.
The integration of the function of a STEM and an SEM in a single particle beam apparatus is known. It is therefore possible to carry out examinations of objects with an SEM function and/or with a STEM function using this particle beam apparatus.
Furthermore, the prior art has disclosed the practice of analyzing and/or processing an object in a particle beam apparatus using, on the one hand, electrons and, on the other hand, ions. By way of example, an electron beam column having the function of an SEM is arranged at the particle beam apparatus. Additionally, an ion beam column is arranged at the particle beam apparatus. Ions used for processing an object are generated by means of an ion beam generator arranged in the ion beam column. By way of example, material of the object is ablated or material is applied onto the object during the processing. The ions are used, additionally or alternatively, for imaging. The electron beam column with the SEM function may serve, in particular, for examining further the processed or unprocessed object, but also for processing the object.
The aforementioned particle beam apparatuses of the prior art each have a specimen chamber in which an object that is to be analyzed and/or processed is arranged on a specimen stage. It is furthermore known to arrange a plurality of different objects simultaneously at the specimen stage so as to analyze and/or process them one after the other using the respective particle beam apparatus that has the specimen chamber. The specimen stage is embodied to be movable so as to position the object or objects in the specimen chamber. A relative position of the object or objects with respect to an objective lens is set, for example. A known specimen stage is embodied to be movable in three directions which are arranged perpendicular to one another. Moreover, the specimen stage can be rotated about two rotational axes which are arranged perpendicular to one another.
The aforementioned particle beam apparatuses of the prior art have at least one of the following units for adjusting the particle beam, i.e., for shaping the beam of the particle beam and/or for setting the beam direction of the particle beam: a displaceable aperture unit, an electrostatic deflection unit and a magnetic deflection unit.
The objective lens of the known SEM is discussed in more detail below. The objective lens of the known SEM has pole pieces, a bore being embodied therein. A beam-guiding tube is guided through this bore. At a first end, the beam-guiding tube has an anode, which is arranged lying opposite an electron source. The electrons of the primary electron beam are accelerated to the anode potential due to a potential difference between the electron source and the anode. By way of example, the anode potential is between 1 kV and 20 kV in relation to a ground potential of a housing of the SEM. Further, a coil for generating a magnetic field is arranged in the pole pieces. Moreover, the known objective lens may comprise a termination electrode, which has a first side and a second side. The first side of the termination electrode is directed in the direction of the object. The second side of the termination electrode is directed in the direction of a tube electrode, which forms a second end of the beam-guiding tube. The termination electrode and the tube electrode form an electrostatic retardation device. This is because provision is made in the known objective lens for the tube electrode, together with the beam-guiding tube, to lie at the potential of the anode of the SEM, while the termination electrode and the object in the SEM lie at a lower potential in relation to the potential of the anode. By way of example, this can be the ground potential of the housing of the SEM. As an alternative thereto, the object and the termination electrode can also lie at different potentials; however, these are both lower than the potential of the anode. Consequently, the known objective lens has a first electric field between the beam-guiding tube and the termination electrode and a second electric field between the termination electrode and the object. On account of the electrostatic retardation device, the electrons of the primary electron beam are decelerated to a desired energy that is required for examining the object.
In order to obtain good imaging of the object—i.e., imaging with a good resolution and a desired contrast—with the known SEM, the object should be aligned by means of the specimen stage in such a way that the second electric field between the object and the termination electrode is as rotationally symmetric as possible. If the area of the object to be imaged with the SEM is virtually plane, this area should be aligned parallel to the termination electrode in order to achieve the aforementioned object. However, as a rule, the area of the object to be imaged is not plane. In order to obtain good imaging in this case, too, the practice of sweeping an object voltage applied to the object and, at the same time, aligning the object by tilting the specimen stage while this object voltage is swept is known. Here, sweeping the object voltage of the object is understood to mean that the object voltage applied to the object is set to an object voltage value and this object voltage value is subsequently changed periodically. During the aforementioned sweeping of the object voltage, the object is aligned in such a way by tilting the specimen stage that either the image of the object displayed on the display device does not move or any such movement of the displayed image has a minimal deflection. The above-described procedure, i.e., sweeping the object voltage applied to the object and, by tilting the specimen stage, aligning the object into a position in which the image has a minimum deflection or does not move, leads to the deflecting effect of the second electric field between the termination electrode and the object being neutralized.
However, the above-described procedure does not take account of, firstly, the first electric field between the beam-guiding tube and the termination electrode and, secondly, the magnetic field generated by the objective lens. However, the first electric field and the magnetic field should also be taken into account; otherwise, the primary electron beam is deflected by both the first electric field and the magnetic field in such a way that good imaging is not obtainable. The practice of sweeping the cathode voltage of the electron source for the purposes of taking account of the first electric field is known. Expressed differently, the cathode voltage is set to a cathode voltage value. Subsequently, the cathode voltage value is periodically changed. During the aforementioned sweep of the cathode voltage, the primary electron beam guided in the direction of the object is aligned by displacing the aperture unit and/or by a deflection by means of an electrostatic and/or magnetic deflection unit in such a way that either the image of the object displayed on the display device does not move or any such movement of the displayed image has a minimal deflection. As an alternative thereto, the objective lens current is swept for the purposes of taking account of the magnetic field. Expressed differently, the objective lens current of the objective lens is set to a current value. Subsequently, the current value is periodically changed. Here, too, during the aforementioned sweep of the objective lens current, the primary electron beam guided in the direction of the object is aligned by displacing the aperture unit and/or by a deflection by means of an electrostatic and/or magnetic deflection unit in such a way that either the image of the object displayed on the display device does not move or any such movement of the displayed image has a minimal deflection. However, neither the first electric field nor the magnetic field always has the same axis of symmetry on account of mechanical tolerances in the SEM and on account of magnetic inhomogeneity. Therefore, the two aforementioned fields both deflect the primary electron beam, in each case on their own. Therefore, good imaging with a desired resolution and with a desired contrast is not obtainable despite the above-described procedure since the primary electron beam extends neither along a desired axis of symmetry of the first electric field nor along a desired axis of symmetry of the magnetic field.
If an objective lens in the form of an electrostatic round lens is manufactured perfectly, the latter has one axis of symmetry, namely the rotation axis. An electron of a primary particle beam moves without being deflected along this axis of symmetry. However, producing a perfectly manufactured electrostatic round lens is difficult. Electrostatic round lenses are often not manufactured perfectly. Therefore, an axis of symmetry denoted as an axis of symmetry of these electrostatic round lenses is often only a desired intended axis of symmetry which does not really exist in the actually produced electrostatic round lenses. In the case of a magnetic round lens, as a rule, inhomogeneities in the magnetic material cause the magnetic field not to be symmetrical in relation to the desired intended axis of symmetry. The following is obtained during sweeping: If an electron of the primary electron beam is situated at a start point in the region between the objective lens and the electron source, the direction at which the electron enters the objective lens can be adjusted by means of the aperture unit and the electrostatic and/or magnetic deflection units. If the integrated deflection that is caused by the entire objective lens is zero at the object, the objective lens current can be swept without an impact point of the electron on the object changing in a linear and quasi-static approximation. In practice, there are minor changes around the impact point in that case.